The present invention relates in general to optical communication systems, and is particularly directed to a new and improved, electronically agile, free-space optical communication apparatus, that is configured to provide for selectively directing each of a plurality of independent optical beams, such as those modulated with respectively different communication signals, through a common optical aperture in respectively different directions to a plurality of spatially diverse receiver sites.
Currently available optical (e.g., laser-based) communication systems intended for free space applications, such as building-to-building local area networks and trunk extension systems, are customarily configured as (short range and long range) free space xe2x80x98point-to-pointxe2x80x99 systems. As shown diagrammatically in FIGS. 1 and 2, such systems typically include local and remote optical (laser-based) transceiver pairs 1/2 and 4/5, each of which has an associated telescope for an aperture, and are optically coupled to one another over one or more line-of-sight optical links 3/6.
As further shown in FIG. 2, for long range applications in excess of a few km, some form of actively driven mechanical stabilization platform 7 is customarily used to maintain beam pointing. In addition, for point-to-point applications that are consistent with hub-spoke operation, the systems have a highly integrated configuration, such as that shown in FIG. 3, and require a substantial amount of hardware to provide multiple point-to-point links between a high power hub site 8 and a plurality of receiver (subscriber) sites 9. Unfortunately, none of these existing architectures addresses tactical applications or mobile nodes, nor do they provide for low cost point-to-multipoint communications.
In accordance with the present invention, advantage is taken of recent and emerging technology developments in free-space optical communications (FSOC), including economically produced dense arrays of addressable transmitter and receiver elements, to provide an lectronically agile multi-beam optical transceiver (or xe2x80x98AMOXxe2x80x99) for use in a point-to-multipoint hub, that allows any of multiple optical beams (independently modulated with respectively different communication signals), to be dynamically routed and spatially re-directed, as desired, in respectively different directions through a common optical aperture over a relatively wide field to a plurality of spatially diverse sites or nodes. The invention also includes a tracking array that actively corrects for pointing and tracking errors that may be due to relative node motions and atmospheric induced distortions. Being electronically agile, the invention has no moving parts, and thus achieves a reduction in size, weight, and cost, while improving reliability and functionality.
To this end, a multiport input-output unit contains an input crossbar switch, respective inputs of which are supplied with electronic signals, such as subscriber signals supplied by way of a digital telecommunication network. The crossbar switch""s outputs are connected to respective transmitter driver circuits coupled to a (two-dimensional) array of light emitter (laser) elements, whose output beams are coupled to a telecentric lens system. For an integrated transceiver application, the telecentric lens system contains a frequency-selective (dichroic) interface that allows light at the transmission wavelength generated by the light emitter array to pass to and diverge from a convex face of the lens, whereas light incident upon the lens""s convex face is reflected by the dichroic interface to an opto-electronic receiver array.
The telecentric lens performs a geometric transform of a beam from a spatial location of the transmit array along a path passing through a focal point within an aperture at the exit face of the lens diverges in accordance with the two-dimensional spatial displacement from the beam axis of its associated emitter within the transmitter array. This means that the desired travel path of an optical beam carrying a particular signal channel may be readily defined by controlling the crossbar switch feeding the two-dimensional transmitter array. Thus, the invention is able to project multiple transmit optical signals from a two-dimensional planar array of optical emitters into differentially divergent, free-space beams through a commonly shared aperture of the telecentric lens, with a precise relationship between the position of an emitter and it""s angular transmit direction.
In the receive or return path direction, the telecentric lens accepts multiple receive optical beams and directs them onto a two-dimensional receiver array. The optics of the lens system produce a typical Fourier transform operation, and the focal plane positions correspond to unique angular beam directions. The photodetector array has its outputs connected to respective signal demodulators outputs of which are coupled to an receiver side crossbar switch, outputs of which are supplied to digital subscriber lines coupled to the transmit crossbar switch.
An auxiliary tracking (two-dimensional) photodetector array may be used to monitor one or more beams from nodes whose spatial locations relative to the hub site are precisely known. Any offset in the spatial location of a xe2x80x98trackingxe2x80x99 beam from such a node on the tracking array is used as an error correction signal by the control processor to impart the appropriate (X-Y) correction, as needed, in the steering commands supplied to the crossbar switches so as to provide for real-time pointing/tracking and atmospheric correction capability.
In some applications, the transmit and receive beams may be split between two spatially separate apertures, so that (transmit vs. receive) wavelength segregation is not necessary. Potential advantages of such beam division include larger receiver apertures for improved signal collection, optimization to specific transmit and receive array configurations, and a reduction in the complexity of diffractive optical elements or holographic optical elements.
The transmitter array may be implemented in a variety of ways. Where the number of remote nodes, which are generally spatially stable, is small, a sub-populated non-switchable or xe2x80x98non-agilexe2x80x99 array may be employed. An example of a xe2x80x98non-agilexe2x80x99 application involves the use of an Ethernet network to xe2x80x98locallyxe2x80x99 connect buildings that are reasonably close to one another. A limited set of discrete laser sources may be hard-wired via an array of associated optical fibers to respective spatial locations within a light emitter array plane, for which the spatial-to-angular transform produced by the telecentric lens will direct the emitter beams along angular directions to subscriber nodes.
Although the invention may be applied to such xe2x80x98non-agilexe2x80x99 multi-beam terminals, the preferred embodiment of the invention employs the xe2x80x98agilexe2x80x99 configuration described above, in which any array position is potentially active and dynamically addressable. A non-limiting application of an agile array would be to allow mobile communication personnel to rapidly deploy a local area network (LAN), while providing for dynamic variations in the number and/or physical locations of the nodes of the network, and to track and correct for relative motion between the nodes.
To realize cost-effective, agile transmitter arrays, vertical cavity surface-emitting laser (VCSEL) components may be employed in combination with an Mxc3x97N digital crossbar switch. Alternatively, the VCSELs may be replaced by discrete laser diodes in a sub-populated array. An advantage of VCSELs is their ability to simultaneously emit multiple transverse modes (MTMs). A multi-transverse mode source may reduce the effects of atmospheric scintillation in a FSOC link. With an MTM source, the beam is already somewhat homogenized, so that additional phase scrambling due to scintillation may be greatly reduced. This effect may also be generated or enhanced by using a custom-designed optical element to scramble the phase-fronts prior to transmission. As a non-limiting example, a DOE/HOE or a simple diffuser may be employed. This technique may also be used to produce the desired beam angle for the intended application.
As an alternative to electronic configurations, each crossbar switch may be implemented as an all-optical fiber optic switch. A principal advantage of an optical fiber approach is that the number of laser elements can be reduced to match the number of input signals. The transmit element array may comprise a fully populated fiber optic bundle, which can be configured and sized to have the desired element center-to-center spacing.
Although the transmitter array may comprise a spatially periodic, two-dimensional array of point-source emitters, the beams impinging upon the receiver array can be expected to be incident at arbitrary locations within the array depending on the angular position of subscriber nodes. The receiver array elements should therefore have the largest possible active area (up to the desired spatial resolution of the array) and the highest possible fill-factor (or very little dead space between photodetector elements). Also, the node connecting the detector, preamplifier, and feedback resistor components of a respective photodetector element must be relatively xe2x80x98physically shortxe2x80x99 in order to preserve the receiver""s bandwidth performance. In a two-dimensional receiver array, this node length may become unacceptable due to the loss of the second dimension for mounting components. The receiver array may be configured as a fiber bundle outputs of which are (optical-fiber) routed via a set of fiber optic switches to a subset of optimized discrete photodetectors.